Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous compounds, which may include nitrous oxides (NOx).
Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of NOx emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. One method that has been implemented by engine manufacturers to comply with the regulation of these engine emissions has been to develop homogeneous charge compression ignition (HCCI) engines that burn compressed natural gas (CNG).
HCCI combustion takes place spontaneously and homogeneously without flame propagation. In the HCCI engine, fuel is homogeneously premixed with air, but with a high ratio of air to fuel. When the piston nears top dead center (TDC) of the compression stroke, compression heating of the piston within the cylinder causes this mixture to auto-ignite (spontaneously combust). The homogeneous CNG/air mixture combined with a lean-burn process produces a lower local flame temperature, which lowers the amount of NOx production.
One problem associated with HCCI engines includes control of combustion timing. In particular, specific quantities of fuel and air in the combustion chamber must be precisely maintained in order to ensure auto-ignition at an intended timing. This need for precision leads to transient control problems, requiring the system to ascertain initial cylinder conditions and then to estimate temperature later in the cycle based on measured cylinder pressures. High cost instrumentation and computing power may be required to perform these high-speed complex processes.
As disclosed in U.S. Pat. No. 6,640,773 (the '773 patent), issued to Ancimer et al. on Nov. 4, 2003, combustion control of the homogeneous CNG/air mixture may be accomplished with a dual-fuel system. The '773 patent describes introducing a gaseous main fuel (CNG, propane, bio-gas, landfill gas, or hydrogen gas) and air into the cylinder to form a substantially homogeneous mixture prior to combustion. The '773 patent further describes introducing a pilot fuel (diesel fuel or dimethylether) that ignites more readily than the main fuel into the cylinder near top-dead-center (TDC) of the compression stroke. Upon auto-ignition of the pilot fuel, the temperature and pressure of the main fuel rapidly increase to cause auto-ignition of the main fuel. In this manner, injection of the pilot fuel controls auto-ignition timing of the main fuel.
Although the apparatus of the '773 patent may improve control over combustion timing of an HCCI engine, the apparatus requires two separate fuel systems, one for the main fuel and one for the pilot fuel. The two separate fuel systems may increase the cost and complexity of the HCCI engine. In addition, the benefits of using the low NOx-producing main fuel may be diluted by using the higher NOx-producing pilot fuel.
The disclosed homogeneous charge combustion control system is directed to overcoming one or more of the problems set forth above.